The delayed-type hypersensitivity reaction of Allergic Contact Dermatitis (ACD) can be acquired when a sensitized individual later becomes challenged with the same small molecule. ACD manifests itself during the phase of elicitation; following penetration of the epidermis and acquisition/processing by an antigen presenting cell (APC)—a specialised cell within the skin, which presents the allergen or antigen to other cells known as T cells recruited by chemokines to the skin, causing their activation and the production of high levels of lymphokines. These molecules give rise to a secondary response with skin inflammation and keratinocyte (skin cell) apoptosis. Distinct from its near relative, Irritant Contact Dermatitis (ICD), which is caused by irritants (e.g. soap, detergents, perfumes etc) and which can affect anyone who succumbs to sufficient exposure, ACD is influenced by environmental and genetic factors and may take many years to manifest, long after initial contact. With approximately 20% of the general adult population believed to be allergic to one or more chemical sensitizers, and with a growing list of novel cosmetic and pharmaceutical products becoming available, ACD threatens to be an increasing future occupational and consumer health problem. Developing suitable and sensitive methods for the assessment of a chemical's potential to cause ACD will be a crucial step in combating this disease. As regards drug allergies, these are rarely detected in non-clinical studies and are usually only observed in Phase 3 clinical trials or during commercialization when larger populations are exposed to the drug. Although the number of drugs that elicit allergic reactions is relatively low, the potential impact is very high due to the late stage of development in which it is detected. Therefore, non-clinical methods to predict for the potential to produce allergic reactions are needed to help in compound selection.
Identifying chemicals that have the potential to induce hypersensitivity skin reactions is a mandatory component of new product discovery by pharmaceutical and cosmetic industries. Historically, predictive testing has exclusively relied on in vivo animal testing. In the traditional guinea pig test, the product is painted on the body and the guinea pig is then injected with an additional chemical to help accentuate the effect of the test chemical in developing dermatitis. Alternatively in the mouse ear swelling test, the mouse's ears are painted with the test substance and its immunological response is determined by examination of lymph node tissue. However, with an imposing EU ban on animal testing nearing its full implementation, there is a pressing need for the development of alternative predicative in vitro and in silico techniques. Although it is known from the prior art to gage up and/or down regulation of gene products such as cytokines these assays are laborious and results are inconsistent. No validated in vitro model currently exists to predict immunogenicity and hypersensitivity or allergic reactions to potential therapeutic compounds, cosmetics and chemical sensitizers.
There is therefore a need for an in vitro assay to discriminate between sensitizers and non-sensitizers for predicting the sensitizing nature of novel pharmaceutical, cosmetic and chemical products. There is a need for a simple, robust, accurate assay for testing novel compounds for hypersensitivity and allergic reactions.